Two principal capacitive sensing and measurement technologies are currently employed in most capacitive touch sensing devices. The first such technology is that of self-capacitance. Many devices manufactured by SYNAPTICS™ employ self-capacitance measurement techniques, as do integrated circuit (IC) devices such as the CYPRESS PSOC.™ Self-capacitance involves measuring the self-capacitance of a series of electrode pads using techniques such as those described in U.S. Pat. No. 5,543,588 to Bisset et al. entitled “Touch Pad Driven Handheld Computing Device” dated Aug. 6, 1996.
Self-capacitance may be measured through the detection of the amount of charge accumulated on an object held at a given voltage (Q=CV). Self-capacitance is typically measured by applying a known voltage to an electrode, and then using a circuit to measure how much charge flows to that same electrode. When external objects are brought close to the electrode, additional charge is attracted to the electrode. As a result, the self-capacitance of the electrode increases. Many touch sensors are configured such that the grounded object is a finger. The human body is essentially a capacitor to a surface where the electrical field vanishes, and typically has a capacitance of around 100 pF.
Electrodes in self-capacitance touchscreens and/or touchpads are typically arranged in rows and columns. By scanning first rows and then columns the locations of individual disturbances induced by the presence of a finger, for example, can be determined.
Typically, rows and columns of electrodes in self-capacitance sensing devices such as touchscreens or touchpads comprise electrically conductive traces or strips of indium tin oxide (“ITO”) laid down on a glass or plastic substrate.
During and after the process of forming such traces on a suitable substrate, defects in such traces or strips will arise, at least in some of the self-capacitance sensing devices. Common defects in ITO traces in touchscreens include shorting between traces, shorting between one or more traces and ground, broken traces, traces that are too thin, too narrow, too thick or too wide, unintended irregularities in the geometries of individual traces, and the like.
Because the foregoing and other defects in ITO traces can significantly affect the performance of a touchscreen or touchpad, testing is often carried out on individual self-capacitance sensing devices after the manufacturing process has been completed. Once such testing method for self-capacitance touch sensing devices is described in U.S. Patent Publication No. 2008/0278453 to Reynolds et al. entitled “Production Testing of a Capacitive Sensing Device.”
There are several problems with testing the integrity of ITO or other types of electrodes in a self-capacitance sensing device, however, such as the need to provide by relatively complicated and time-consuming means precise external stimuli to different locations of a touchscreen to mimic a users touch at predetermined locations thereof, the relatively small changes in self-capacitance that occur as a result of broken or otherwise defective traces, and the small changes in self-capacitance that normally occur along the length of a given electrode trace that is in good operating order. In addition, self-capacitance sensing devices are difficult to test in the field owing to the need to provide the precise external stimuli described above. As a result, self-capacitance sensing devices are very difficult to test in the field.
The second primary capacitive sensing and measurement technology employed in capacitive touch sensing devices is that of mutual capacitance, where measurements are typically performed using a crossed grid of electrodes. See, for example, U.S. Pat. No. 5,861,875 to Gerpheide entitled “Methods and Apparatus for Data Input” dated Jan. 19, 1999. In mutual capacitance measurement, capacitance is measured between two conductors, as opposed to a self-capacitance measurement in which the capacitance of a single conductor is measured, and which may be affected by other objects in proximity thereto.
In some mutual capacitance measurement systems, an array of sense electrodes is disposed on a first side of a substrate and an array of drive electrodes is disposed on a second side of the substrate that opposes the first side, a column or row of electrodes in the drive electrode array is driven to a particular voltage, the mutual capacitance to a single row (or column) of the sense electrode array is measured, and the capacitance at a single row-column intersection is determined. By scanning all the rows and columns a map of capacitance measurements may be created for all the nodes in the grid. When a user's finger or other electrically conductive object approaches a given grid point, some of the electric field lines emanating from or near the grid point are deflected, thereby decreasing the mutual capacitance of the two electrodes at the grid point. Because each measurement probes only a single grid intersection point, no measurement ambiguities arise with multiple touches as in the case of some self-capacitance systems. Moreover, it is possible to measure a grid of n×n intersections with only 2n pins on an IC.
What is needed is a capacitive measurement system that may be employed in touchscreen or touchpad applications that may be tested for trace integrity and proper operation after the touchscreen or touchpad manufacturing process has been completed, as well as after the device has been incorporated into or operably connected to an electronic device that is fast, accurate and of low cost.